How the Fuel Injection System Works

The injectors lie at the end of a series of parts that deliver fuel in an atomised state into the cylinder bore for combustion. Here’s a brief run-down of how the fuel injection system works:

Fuel Pump

The fuel pump is very well named. It pumps the fuel, drawing non-pressurised fuel from the fuel tank. The fuel pump then takes the low pressure input and runs it through a valve where it is compressed by a piston. It then sends this high pressure fuel into the fuel rails that feed each injector.

The Injector

Once pressurised fuel reaches the injector, the onboard engine computer controls the fuel injection. Fuel runs through the injector, which is an electronic solenoid, several times a minute, depending on engine revolutions.

As it is now pressurized, when the solenoid opens, the fuel rushes through the injector and turns from a liquid state to an atomised state, where the fuel is more of a mist. This mixture is more readily ignited in the fuel combustion chamber.

Why use injectors?

Injectors are more precise and efficient than carbureted fuel delivery systems. That is because they offer:

Adjustable Fuel Quantity

If a fuel injector constantly sprayed out fuel, the cylinder could become hydrolocked, where the cylinder floods with too much fuel and the combustion cycle is halted. It could also cause over-firing and damage to the engine bore.

Knowing this, the fuel injector has been designed to pulse, operating in an on-off cycle. This allows for more control over the fuel quantity, as an engine has different fueling requirements at different speeds and operating conditions.

This fine control over fuel quantity helps to save fuel when compared to a carburetor, which simply approximates the engine air-fuel mixture. Injectors can improve throttle response and peak power levels.

Adjustable Fuel Injection Timing

The timing or gap between each injection cycle can also be closely monitored and controlled by the cars on board ECU or ECM, the engine control unit or engine control module. Timing can be adjusted on the fly, again to enhance the operation of the engine and optimize its performance. This greatly increases engine efficiency.

Injectors serve an incredibly important role in the running of your engine. Without them, the engine wouldn’t operate and when one is dirty or clogged, the engine will not run as well as it possibly could.

Symptoms of a Bad Fuel Injector

Now that you know what an injector does and how it works, you can understand how a misfiring or clogged one will negatively impact the engine. Injector ends can become clogged or dirty so that the spray pattern from the nozzle is uneven.

There are numerous symptoms that you’ll notice if your injectors become faulty, dirtied up or clogged:

1. Starting issues

One or more injectors are not delivering the precise combination of fuel and air needed to ignite the engine. The result is a laboured and slow start up, with the engine kicking over more than normal, accompanied by coughing like noises

2. Poor or Lumpy Idle

When idling, engine revs should sit at a more-or-less constant. They may vary a little as the alternator kicks in on modern cars, or wander around some if your belts need looking at, but if the engine sounds like it’s going to stall or has a real trouble running at what is really the lowest possible speed it can, one or more of your injectors may need attention.

3. Failed Emissions / Smoke from Tailpipe

Annual government car tests now include mandatory emission level evaluations. These work on a sliding scale, with older cars only required to meet the standards of the time they were built. If your car fails these tests, it could be caused by a misfiring or clogged injector that is producing excess combustion by-products that are passing out the exhaust.

This may also be accompanied by a thick black smoke from the tailpipe.

4. Poor Performance

Poor performance, whether on acceleration or driving up hills can be caused by injector problems. Remember how we said the amount of fuel injected is adjusted depending on engine loads?

Well this is what happens when the engine tries to adjust to the current demands but is unable. You may also notice this when speeding up on highway entry ramps and the engine is hesitant to reach high revs.

5. Sudden Increase in Fuel Consumption

Because your injectors are not working as they should, an engine begins to waste fuel, either by over compensating or achieving a less complete burn in the cylinder.

Any sudden increase in fuel consumption should be investigated.

6. Surging or Bucking under loads

If your engine bucks and roos as you apply more throttle, this could point to failing injectors

7. Knocking noises / Pre-detonation

Misfuelling issues will cause the fuel in the cylinder to combust at random times. This will cause excess noise, like a knocking or banging noise from deep within the engine. Listen out for that as it places additional stress on surrounding parts.

Leaking Injectors symptoms:

Leaking injectors allow a fine mist of fuel to get into the combustion bore in addition to what comes out of the injector nozzle. This can cause a lot of the above issues but also:

Strong fuel smell in car, from tailpipe

As excess fuel cannot be burned, it passes out through tailpipe. Some of the odours might be noticeable in the passenger area.

When changing oil, you might notice oil has become contaminated with fuel, this causes a thinning of the fuel and could cause terminal engine failure.

When an injector body is cracked, it must be replaced.

How to check for injector problems?

Mechanics use injector testers and a stethoscope to listen for fuel injectors misfiring or not firing at all. They also attach a pressure gauge on a port usually on the fuel rail. If an injector is leaking the system will not hold pressure.

For home car maintenance, you can use a long screwdriver. Place the the tip of the screwdriver above the injector port and place your ear on the handle. You should be able to hear a clear and consistent ticking as the injector solenoid moves up and down. If this is intermittent or you can’t hear anything at all, the injector is bad.